Blood test
Thyroid Function Test
A thyroid function test is a blood test that checks how well your thyroid gland is working by measuring thyroid hormones. It diagnoses an underactive or overactive thyroid.
Quick answer
Thyroid Function Test: what it is and what the results mean
A thyroid function test measures hormones — mainly TSH and thyroid hormone — that show whether the thyroid is producing the right amount.
- Why it is done: It is used when symptoms suggest a thyroid problem (such as tiredness, weight change, feeling cold or hot, or a racing heart) and to monitor thyroid conditions and their treatment.
- Understanding results: A high TSH usually points to an underactive thyroid, and a low TSH to an overactive one, interpreted with the thyroid hormone level and your symptoms.
What it is
A thyroid function test measures hormones — mainly TSH and thyroid hormone — that show whether the thyroid is producing the right amount.
Why it is done
It is used when symptoms suggest a thyroid problem (such as tiredness, weight change, feeling cold or hot, or a racing heart) and to monitor thyroid conditions and their treatment.
What to expect
It is a standard blood test from the arm; no special preparation is usually needed. If you take thyroid medication, you may be advised when to take it around the test.
Understanding the results
A high TSH usually points to an underactive thyroid, and a low TSH to an overactive one, interpreted with the thyroid hormone level and your symptoms.
Good to know
Risks and limitations
Risks are those of any blood test. Results can be affected by other illnesses and some medicines, so they are interpreted in context.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
Thyroid Function Test: frequently asked questions
What does a thyroid function test show?
It shows whether your thyroid is underactive, overactive or normal, by measuring thyroid-related hormones in the blood.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Royal College of Radiologists / relevant professional body
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